It is common practice in many circumstances, notably including coal mines, to use vehicles which are electrically powered and to supply the vehicles with power through a trolley wire and a current collector, or trolley shoe, carried by the vehicle. The current collector slides or rides along the energized trolley wire which is commonly mounted overhead. Relatively high current flows in the trolley wire when one or more vehicles are moving.
The trolley wire is commonly supported at its ends by a structure known as a section insulator which has blocks of insulating material supporting aligned but longitudinally spaced apart sections of trolley wire, leading the shoe or current collector from one energized trolley wire at one side of the section insulator to another energized wire, to a grounded section of trolley wire, or to an unenergized section on the other side. A long and complex network of trolley wires may be divided into separately powered sections by such a structure. In either case, but particularly with one side of the section insulator grounded, a large potential difference can exist across the gaps existing between longitudinally spaced ends of the trolley wire sections. This circumstance can lead to the production of an arc across the gaps, especially when the traveling shoe bridges the gaps as it runs across the section insulator. A structure of this general type is shown in U.S. Pat. No. 2,492,300, Lewis.
As will be recognized, arcs are characterized by high current density accompanied by ionization and the production of significant heat. The direction in which the arc travels is highly variable but because of the heat generated the arc often tends to rise toward the insulating structure of the section insulator and, in doing so, it has a highly detrimental effect on the structure, drastically shortening its useful life.